Fire pressure switches are used to sequentially fire charges during the completion of a gas or oil well. After the initial well is drilled, production can be increased by fracturing the formation using directed or shaped charges. To accomplish the fracturing (“fracing”) of the well, a string of shaped charges is fed into the well to the point where fracing is desired. Each charge is set off in sequence by an electrically triggered detonator which is electrically connected to the surface. Typically, the charges are triggered by alternating positive and negative voltages with appropriately configured diodes so that only the desired charge is triggered. After a particular charge is detonated, the next charge is moved closer to the opening of the well and the process is repeated.
The undetonated charges must be isolated from any water or other fluids released from the previous charge. It is also necessary to isolate the charges electrically from each other so that only the last one in a string is detonated and so that after each charge is detonated, the next one is electrically connected so that it can be detonated. The fire pressure switch uses the pressure pulse from the detonation to move a piston towards a plunger, breaking one electrical circuit and making the next. The newly completed electrical circuit allows the next detonator in the string to become active.
To accomplish this result, three different fire pressure switches are used: a double diode, positive diode and negative diode. The first charge in the string uses a double diode pressure switch. After the first charge, the remaining charges in the string use alternating positive and negative diodes so that each charge is properly detonated in sequence.
The basic arrangement to which the invention disclosed herein is directed is described in U.S. Pat. No. 4,234,768 which is incorporated herein as part of the background of the invention. U.S. Pat. No. 5,531,164 discloses another configuration for sequential detonation of explosive charges including the use of fire pressure switches. Titan Specialties is a supplier of oil field tooling, including fire pressure switched. Titan Specialties sells a fire pressure switch have a configuration of components similar to the configuration of components disclosed herein. Including a switch having a case, piston, spring biased dart and insert functioning similarly to the invention. However, the Titan Specialties switches currently available have has reliability issues and alternative switches disclosed in U.S. Pat. Nos. 4,234,768 and 5,531,164 can be improved. Such reliability problems can be very expensive because when the fire pressure switch fails, the entire string must be retrieved and the failed component replaced.